1. Field of the Invention
The invention relates to a diagnostic method for a fuel cell such as low-temperature fuel cells including polymer electrolyte fuel cells and the like. More particularly the invention relates to a fuel cell diagnostic method concerned with cross-leak of an electrolyte membrane.
2. Description of the Related Art
A polymer electrolyte fuel cell is formed by a stack that includes membrane-electrode assemblies (MEA) and separators. A membrane-electrode assembly is made up of an electrolyte membrane formed by an ion-exchange membrane, an electrode (anode or fuel electrode) formed by a catalyst layer that is disposed on a surface of the electrolyte membrane, and an electrode (cathode or air electrode) formed by a catalyst layer that is disposed on another surface of the electrolyte membrane. Diffusion layers are provided between the membrane-electrode assembly and a separator disposed at the anode side and between the membrane-electrode assembly and a separator disposed at the cathode side. Each separator has a fuel gas channel for supplying a fuel gas (such as hydrogen) to the anode, and an oxidizing gas channel for supplying an oxidizing gas (such as oxygen, or air in ordinary cases) to the cathode. Each separator further has a coolant channel for conducting a coolant (cooling water in ordinary cases). A fuel cell stack is formed by stacking modules each of which includes at least one cell that is formed by stacking a membrane-electrode assembly and separators. Terminals, insulators, and end plates are disposed on two opposite ends of a cell stack in the cell stacking direction. The cell stack is clamped in the cell stacking direction, and is fixed through the use of fastener members (e.g., tension plates) that extend outside the cell stack in the cell stacking direction, and also through the use of bolts and nuts. In this manner, a stack is formed. On the anode side of each cell, a reaction occurs in which hydrogen is separated into hydrogen ions (protons) and electrons. The hydrogen ions migrate through the electrolyte membrane to the cathode side. On the cathode side, oxygen, hydrogen ions and electrons (i.e., electrons produced on the anode side of the adjacent MEA come to the cathode through the separator, or electrons produced on the anode side of the cell disposed at an end of the cell stack come to the cathode of the cell disposed at the opposite end via an external circuit) react to produce water as expressed below.Anode side: H2→2H++2e−.Cathode side: 2H++2e−+(½)O2→H2O
The electrolyte membrane is required to allow only protons to migrate through the membrane in the direction of thickness of the membrane. In reality, however, a very small amount of hydrogen migrates through the thickness of the membrane from the anode side to the cathode side, or a very small amount of air migrates through the thickness of membrane from the cathode side to the anode side (this phenomenon is termed “cross-leak”). Such passage of hydrogen or oxygen across the membrane results in a reaction between hydrogen and oxygen producing heat. Therefore, the membrane degrades, and the durability and service life of the fuel cell are reduced. Two diagnostic methods for determining the presence/absence of cross-leak and the progress thereof have been proposed.
(1) A method is disclosed in Japanese Patent Application Laid-Open Publication No. 9-27336 in which the amount of cross-leak is determined from the change in the cell voltage that occurs when a fuel gas is supplied to the anode side and an oxidizing gas is supplied to the cathode side.
(2) A method is known in which the two electrodes are filled with an inert gas, such as nitrogen or the like, with a differential pressure between the two electrodes, and the amount of pressure change per unit time is measured as an amount of cross-leak.
However, both methods have problems. The first method lacks quantitative performance. In the second method, determination of the amount of cross-leak of each cell is not possible if cells are in a stacked state. To determine the amount of cross-leak of each cell of a stack in the second method, the stack must be disassembled into individual cells for separate measurement.